Somatostatin is a cyclic tetradecapeptide originally isolated in the hypothalamus [Burgus et al., Proc. Natl. Acad. Sci. USA, 1973, 70, 684-688]. The somatostatin regulating mechanism commences by means of binding to the G protein-coupled sstr1, sstr2, sstr3, sstr4 and sstr5 somatostatin receptors [Patel et al., Front. Neuroendocrinol., 1999, 20, 157-198]. They all bind to somatostatin with nanomolar affinity [Patel et al., Endocrinology, 1994, 135, 2814-2817]. The five somatostatin receptors differ in their distribution in tissue and pharmacological properties. The first known action of somatostatin is the inhibition of secretion of the growth hormone via the sstr2 and sstr5 receptors. Furthermore, somatostatin inhibits glucagon secretion through sstr2 and insulin secretion via sstr5 [Strowski et al., 2000, Endocrinology, 141(1), 111-117]. The sstr3, and to a lesser extent, sstr2 receptors seem to be involved in induction of cell apoptosis [Qiu et al., 2006, World Gastroenterol., 12(13), 2011-2015]. In addition, sstr1 and sstr5 have an inhibitory effect on the cell cycle and sstr1 could modulate angiogenesis [Bocci et al., Eur. J. Clin. Invest., 2007, 37(9), 700-708]. The function of sstr4 has been studied less, although recent studies have shown its potential as a therapeutic target in hepatic diseases and prostate cancer [Jung et al., Laboratory Investigation, 2006, 86, 477-489; Hansson et al., Prostate, 2002, 53(1), 50-59].
In clinical practice, somatostatin is used as therapy for the treatment of gastrointestinal bleeding due to esophagogastric varices and as an adjuvant in the treatment of secreting pancreatic fistulae. The lack of side effects is its greatest advantage. Despite its biological profile, one of the drawbacks of somatostatin is its short blood half-life (less than 3 min), which makes continuous endovenous infusion necessary and restricts its use to a hospital level.
The short half-life of somatostatin brought about the development of analogs that present greater stability against enzymatic degradation. Somatostatin analogs which maintain the structure of the original molecule can be found in the state of the art. For example, U.S. Pat. No. 4,211,693 A describes somatostatin analogs in which any of the phenylalanine amino acids has been substituted with para-halogenated or para-methoxylated phenylalanine, and U.S. Pat. No. 4,133,782 A describes somatostatin analogs in which the tryptophan amino acid in position 8 is the D-stereoisomer. Brown et al. also describe primarily analogs with D-amino acids [Brown et al. J Physiol. 1978, 277, 1-14]. Besides these initial proposals for somatostatin analogs from the original molecule, most of the works known in the state of the art relate to analogs of 8 or fewer amino acids [Janecka et al. 2001, J. Pept. Res., 58(2), 91-107; Pawlikowski et al., 2004, Curr. Opin. Pharmacol., 4(6), 608-613].
Octreotide was the first analog developed in clinical practice. It has a structure with a 6-amino acid cycle. Other octreotide analogs which maintain a common structure with a 6-amino acid cycle (lanreotide, vapreotide, pasireotide) can be found in the state of the art. The reduction of the original 12-amino acid cycle of somatostatin to a 6-amino acid cycle restricts the flexibility of the original molecule by limiting interaction with some receptors of the family of sstr1-sstr5 receptors. While somatostatin binds with nanomolar affinity to each of its sstr1, sstr2, sstr3, sstr4 and sstr5 receptors, octreotide, lanreotide and vapreotide only bind with high affinity to the sstr2 receptor, with moderate affinity to the sstr5 receptor, and with moderate-low affinity to sstr3 and they do not bind to the sstr1 and sstr4 receptors [Patel et al., Endocrinology, 1994, 135(6), 2814-2817]. In the example of pasireotide, the interaction with the sstr4 receptor is lost and the affinity for the sstr2 receptor is an order of magnitude lower [Weckbecker et al., Endocrinology, 2002, 143, 4123-4130].
The identification of different expression profiles of the five somatostatin receptors in target organs of somatostatin explains the limited efficacy of treatments with octreotide, lanreotide and vapreotide in pathologies in which the sstr2 receptor is under-expressed [Khare et al., Faseb J., 1999, 13(2), 387-394].
The use of these 3 analogs has been approved only for a limited number of clinical applications, such as acromegaly, metastatic carcinoid tumor, VIPomas, diarrhea, bleeding of esophageal varices and perioperative protection in pancreatic surgery. Taking into account the wide range of pathologies in which the expression of somatostatin receptors has been identified [Pawlikowski et al., Neuro Endocrinol Lett, 2003, 24 (1-2), 21-27; Vaysse et al., Curr. Med. Chem., 2005, 4, 91-104; Reubi et al., Endocr. Rev., 2003, 24(4), 389-427; Kumar et al., Neuroscience, 2005, 134(2), 525-538] these known analogs solve a small area of possible applications.
In this context of clinical interest for new somatostatin analogs with a high affinity for several or all the receptors thereof and of new applications of somatostatin and its analogs [Tulipano et al., Eur. J. Endocrinol., 2007, 156 Suppl 1, S3-S11; Lamberts et al., Eur. J. Endocrinol., 2002, 146(5), 701-705], the heterogeneous expression of the sstr2 and sstr5 receptors in secreting adenomas of the growth hormone and their treatment with bispecific analogs has demonstrated better control of growth hormone hypersecretion with respect to treatment with octreotide and lanreotide, with preferential affinity for the sstr2 receptor, with an inhibitory concentration IC50 of 12 to 18 times greater than sstr5 [Savenau et al., J. Clin. Endocrinol. Metab., 2001, 86, 140-145].
Therefore, there is still a need to find new synthetic somatostatin analogs for the treatment of those pathologies which present expressed somatostatin receptors sstr1, sstr2, sstr3, sstr4 or sstr5 and which present greater stability in blood than somatostatin.
The new somatostatin analogs must present a broader profile of interaction with the somatostatin receptors, if possible a universal profile of interaction with the 5 sstr1 to sstr5 receptors, or which is at least specific for those receptors with which the analogs already known in the state of the art do not interact, such as the sstr1, sstr4, and sstr3 receptors.